The present invention is in the technical fields of optics and ophthalmology. The present invention relates to a system for the measuring of the interpupillary distance through an optical process, without physical contact.
In the prescription of corrective eyeglasses it is necessary to have knowledge of several biometric data regarding the dimensions and shape of both face and the visual system of the patient, including the respective frame supporting the lenses. Knowing these data, it is possible to obtain personalized lenses which are able to correct the patient visual deficiency. Among these biometrical data, one of uttermost importance is the Interpupillary Distance, defined as the distance between the centers of the pupils.
The Interpupillary Distance is a crucial information on the creation of ophthalmic lenses, since it defines the distance from the optical axis of both lenses to place in the frame. A correct measurement of this distance is determinant for a perfectly corrected eyesight. An incorrect measure of this distance (more than 1 mm) will create a misalignment of the lenses relative to the eyes, which may lead to imperfect vision and eye strain.
Further biometrical data relevant for the lens design are: the distance from the center of the pupil to the patient's nose, known as mono-pupillary distance; the distance from the center of the pupil to the lens, traditionally known as the vertex distance and the inclination angle of the lens relative to the optical axis.
Several systems have been developed to measure the Interpupillary Distance, however most of them are complex and costly, and are mainly used by specialized technicians, mostly optometrists and ophthalmologists in specific places, such as clinicals or opticians. These systems are, in general, quite accurate and can take measures with an error of less than 1 mm, they use specialized machinery and typically require extra objects to be placed above the patient's face or over the frame. The most relevant systems and the ones most related to this invention are the following:                a) Optical only systems: One of the most used classical system is the Pupilometer, described in the patent FR1506352. This device, composed of a set of lenses, contains three openings, two of them reproduce the glasses lens structure and shall be placed in contact with the patient nose. The remaining one is placed in the opposite side, from where the Measurer will look, with his eye placed in the focus point of a collimator lens. This lens allows for the light source, contained in the device, to be placed in the infinity, which simulates distance vision. The lens can also be movable, which enables the measurement of the Near Interpupillary Distance. The Measurer can move two markers such that both are aligned with the patient pupils, determining the Interpupillary Distance in that way. These systems are specially constructed for this lengthy process, which requires the active participation of a technician.        
The presented invention uses a different technological principle, since it eliminates the participation of the technician and allows the patient to measure his own Interpupillary Distance using a device that is accessible most of the time, such as a tablet, smartphone or a laptop computer.                b) Optical systems with artifacts over the face or frame: Recently, a new set of commercial systems were presented, which are based in a image sequence, such as the system described in patent WO 2011/042623. Their principles are based on the capture of a sequence of images of the patient's face from different perspectives. It is necessary to place an object with predefined markers above the patient's face or frame. This artifact is crucial to obtain a geometric relationship between the pictures. The present invention, unlike the latter, neither needs any device on the patient's face, nor requires the patient to carry out a set of poses.        c) Optical systems with user movement: The system WO 2011/113936 is also based in a set of photographs, in which the patient makes a predetermined set of movements and, unlike the previous ones, requires no artifact on the face or frame for measuring the Interpupillary Distance. This system applies a statistical calculation which is based on an iterative optimization algorithm for estimating: the distance to the camera, the Interpupillary Distance, the focal length of the camera, the radius of the eyeball and the pupil size, taking into account all images from a sequence of photographs taken from the patient movements. No results are known regarding the exactness on the estimation of this multi variable problem. In addition to the necessary computational burden, this system requires the patient to conduct a predetermined motion, and depends on the automatic detection of the circle around the pupil, which can be difficult since it requires sophisticated methods of detection and a high camera resolution.        
The present invention, unlike the previously described system, requires no patient movement, since the measurement is obtained from a single photography. Additionally, the Distance estimation is based on simple calculations of elementary geometry, requiring no computational burden at all.                d) There are other systems that can locate the pupil three-dimensionally, using stereo vision systems (with multiple cameras or a moving camera) or using active processes for detecting the distance from the patient to the system, such as the patented system WO2009/007731, or the case of systems with structured light scanning, such as CN101739717. These systems can be quite accurate in metric terms, however they use specific hardware devices which are highly calibrated and costly. In some cases they are obtrusive or bothersome to the patient, as is the case of the systems which use structured light systems.        
The present invention uses a different technological principle than these systems: it is non-intrusive and easy to implement in a conventional device, which is available to most people, such as a laptop, tablet or smartphone.
In recent years, the market for online sale of lenses and frames has increased significantly and the offer is diverse. This context arises with a new need: obtaining the Interpupillary Distance of the patient remotely, discarding the inconvenience of paying a visit to a specialized technician. If someone opts to acquire his new glasses in an online retail shop, it is necessary to know what his Interpupillary Distance is previously, and the only reliable way to obtain it is to perform the measurements in an ocular specialist. This inconvenience eliminates the main advantage of online sale: buying without the physical presence of the customer. Thus, to obtain the Interpupillary Distance, some online retailers are asking their customers to apply rudimentary techniques of measurement, based on objects placed in front the patient's face or forehead or based on direct measurement with a ruler. These techniques are based on empirical or approximate geometric principles and therefore produce erroneous results, whose measuring errors may exceed 3 mm. These procedures are generally discouraged by optometrists.
The first of these systems corresponds to a simple direct measurement with a ruler, which involves multiple measurement errors due to the distance from the ruler to the measured object. The errors are significant and difficult to quantify. When the measurement is performed in front of a mirror, the error will be even higher.
There are a few more rigorous systems which use only one camera and an extra object with known marks or dimensions that must be placed over the patient's face. Examples of these systems include objects such as credit cards or target aims to be placed over the patient forehead or above the patient's frame (system proposed in US patent 2011/0267578). These systems assume that the artifact is co-planar with the centers of the pupils, which never happens. Moreover, the method is very dependent on good use of the artifact by the user. Thus, to the measurement error contribute not only parallax errors, but also errors due to frequent misuse of the artifact.
Unlike previous systems, the present invention does not need extra artifacts or rulers on the face of the patient and the results obtained are not an approximation of reality.